1. Field of the Invention
The present invention relates generally to solar distillation systems. More particularly, the present invention relates to a high output solar distillation system that is practical to manufacture and efficient to operate.
2. Discussion of the Related Art
Most fresh water is obtained from rivers or lakes fed by rain or melting ice. When sufficient fresh water is not available; processes using thermal powered water distillation or pressure driven reverse osmosis are often used to convert brackish or salty water to pure water. These processes are expensive to set up and operate. They are less economical and often impractical for very small-scale versions.
Use of the sun to evaporate water, and use of glass or plastic covers to contain and condense the vapor has been long known. Nevertheless, only limited versions of such systems are currently in use. One major problem of previous versions of such systems was the limited amount of water that could be produced in a given area. This resulted in very extensive areas being required for reasonable production levels, and this contributed to the cost and maintenance requirements.
The solar flux is about 1.4 kW/m2 at the earth""s location. Just under 1.0 kW/m2 reaches the ground at midday on a clear day and with a high sun angle, due to atmospheric losses. The effective daily total solar flux to a collector depends on the inclination of the collector, amount of clouds, seas, etc. The following discussion relates to a solar collector design, which uses stationary collectors inclined at an angle close to the average latitude angle, and facing the average noon sun. For this case, the maximum total effective daily solar flux is about 8 kW hours/m2/day. For most of the United States, the year round average is approximately 5 kW hours/m2/day. This energy can be used to evaporate water for purification.
Conventional single stage solar evaporator systems are constructed as enclosed structures that pass sunlight in through a sloping glass or plastic wall, have a dark floor covered with a thin layer of source water, and use radiative, convective, and conductive heat transfer to and through the glass to the outside air. The tilt of the wall is required to drain the condensate to collectors. About 92% of sunlight energy pass through the glass sheet, and the dark floor absorbs about 90% of the remainder. The energy raises the water temperature above the outside temperature. Since the glass is cooler than the water, the water vapor condenses on the underside of the glass surface and drains down to the collector. The absorption and heating effectively converts short wavelength sunlight to long wavelength radiation, much of which is trapped in the enclosure due to the spectral response of the glass. This phenomenon is commonly known as the greenhouse effect. Radiation from the heated water to the condenser surface, and the energy used to raise the water temperature above ambient temperature decreases the available energy, so that the effective energy to evaporate the water is typically only about 60% of the total. Glass is normally preferred to plastic for these types of systems due to the better wetting characteristics of glass. If plastic is used, the tilt angle and average drain distances are limited to a smaller range to avoid drops falling from the sheet. Glass is also more scratch resistant and lasts longer in sunlight than plastic. Plastic is sometimes the better choice, however, if its limitations are not overly restrictive, due to its lower weight and greater break resistance, and also due to its ability to be fabricated in complex shapes.
When the water is heated, some of it evaporates, and this removes about 540 cal/g of energy from the remaining water. It therefore requires 2.38 kW hours to evaporate 1 gallon of water at constant temperature. This means that the maximum of approximately 4.8 kW hours/m2/day that is available could evaporate up to 2.02 gallons per day, with a maximum year-round United States average of about 1.26 gallons per day. Due to other system losses such as heat conduction (including through the edge and backside), convective heat losses, and thermal capacity of excess supply water, the best single effect systems actually produce a peak of about 1.2 gallons/m2/day and an average of about 0.8 gallons/m2/day. Even these levels require a fairly high solar flux region.
The main limitation with the above single effect system is that all of the sunlight energy available goes to heat the water one time, overcome the heat of vaporization, and then the system dumps all of this energy into the air in order to cool and condense the vapor out. This process has very poor thermal efficiency.
Use of multiple effect systems can improve the production over single effect systems. The heat of vaporization used for one stage is recovered during condensation of the distillate and passed on to the next stage closer to the external surface. This multi-stage regeneration process, driven by a continual temperature drop stage to stage, can multiply the pure water production. Reflection, refraction, and scattering of the incoming light from each partition drops the energy reaching the darkened absorbing surface beneath the last partition more than for a single effect version, so the maximum distillate production per effect is decreased. The maximum total production in such a process is also limited by the higher temperatures obtained with such systems. The total production for practical multiple effect systems, however, can greatly exceed single effect systems. Unfortunately, previous versions of multiple effect solar distillation systems are either too inefficient or too complex or costly to be of practical use.
The solar distillation system of the present invention is capable of significantly greater production per area, with an easily installed and maintained structure. The new design can be moved easily, and is practical in small or large-scale systems. The new solar distillation system results in economical purification of water at all scales of operation. In addition, the system can be used to distill other liquids such as ethyl alcohol.
The present approach uses inclined parallel partition surfaces that are spaced a very short vertical distance apart, so that a very compact and strong panel structure results. The small distance between partitions results in diffusion being a major mode of water vapor transport within the chambers. This also results in a minimum temperature drop per stage to achieve the desired level of water vapor diffusion. It should be noted that most previous versions in the literature used fairly large spacing, and this was a major cause of low efficiency. The input sunlight has to pass through all of the partitions to the heat absorbing bottom layer. An insulation sheet is used behind the dark absorbing layer to maximize the energy used to heat the water. As in the single effect system, the usable solar flux energy available is considerably less than 1 kW/m2.
If a 3 effect is used as an example, the maximum available absorbed energy is estimated to be about 450 W/m2 due to additional wall reflection, refraction, and scattering. The maximum temperature of this lower layer depends on the outside temperature, but is typically below 160 degrees F. This energy first evaporates supply water from the bottom of the lowest effect chamber. Since the temperature drops continually from the heat absorber to the exterior, some water condenses on the upper surface of the lowest effect chamber. This condensation gives the 540 cal/g due to the heat of vaporization back to the partition. Water is also run down the top of the second partition, and this water absorbs this energy of condensation. Since the temperature of the second partition is still reasonably high (but lower than the lowest partition), water evaporates from the second partition, and is condensed at the third partition up. The same process continues to the final partition, which now dumps the energy into the surrounding air. If the outer surface is dry, convection and radiation remove the energy. If an external water stream is used on the outer surface, some of the external energy is removed by evaporation. Evaporation insures maximum cooling, even when the exterior air speed is very low. This also lowers the overall device temperature, and also increases light transmission of the surface when it becomes frosted due to sand erosion. The maximum distilled water production for a 3 effect version has been estimated to be about 3 gallons/m2/day for practical designs, with average production of up to 2 gallons/m2/day. This is about 2.5 times the production of a good single effect system. More than 3 effects can be used, but eventually the lower transmission and higher temperatures will limit the maximum number. It is thought that at least 5 effects can be used effectively, where collector area is a major consideration.
Use of glass for the present designs would result in a heavy and fragile structure. The preferred choice is plastic materials. However this presents some special problems for feeding and draining the surfaces properly due to the poor wetting properties of plastic. These problems are eliminated in the present approach. Materials currently commercially available would allow simple and long lived structures to be made. In particular, UV protected polycarbonate can be easily extruded, is strong and light, and can be used for prolonged times in sunlight at the expected maximum temperature. This material is presently made in single and multi-walled sheets and is used for skylights in homes and greenhouses, and lasts for 10 to 20 years.
The main requirements needed to obtain a practical and efficient system with the present approach are: 1) Minimize energy losses; 2) Supply the source water uniformly and controllably; 3) Collect undistilled feed water and distilled water efficiently; and 4) Make a structure that is practical to manufacture.
The first task can be accomplished by insulating water lines and containers, and recirculating the unevaporated feed water supply, to minimize heating excess water (some excess feed water has to be used to carry off concentrated salts and solid residue) The second task uses water pumped through small supply holes feeding narrow water guides to direct the supply water flow. In addition, surfactants are added to make the supply water wet the plastic and minimize deposition of particles. The third and fourth tasks will be discussed next, where two possible versions of structures are described.
One form of present invention uses a monolithic extruded structure as a main component. Four partitions and three effect chambers (layers) are preferred in this embodiment. The effect chambers are held apart by vertical walls which run the entire length of the structure, and which are laterally about 1.7 inches apart. Small (laser drilled) feed holes through the floor of the source guides, or an end mounted chamber with feed holes drips water separately onto each of the inclined source guides. The overall structure is tilted slightly to the side as well as down so that the distilled water condensing on the top of each effect chamber drains at an angle toward the collector guides side, and then drains down the vertical wall into the collectors. At the exit end of the collector guides, plugs and drain lines remove the distilled water, while the excess supply runoff is caught in an end chamber and reused.
A different design uses discrete components that can be used to make a multiple effect system. The structure is somewhat similar to the previous version in that it has source water guides (6 to the inch laterally). It also has taller ribs every inch laterally which are used as spacers in the structure.
The present invention has a hole configuration in which the source water is fed through small holes into each guide. Occasional vent holes allow trapped air to exhaust from the supply volume. When the vent air is gone, some excess water is released into the guides, but this is all recovered later. An alternate design might use a source assembly above the surfaces with short fingers lying in each guide, and with a small hole in the bottom of each finger.
A drain system includes cross drain strips, which are inclined at a small angle, and go from the right edge of the distillation system to a short distance from the left edge. They are placed about 15 inches apart longitudinally. This longitudinal separation was found experimentally to be a reasonable distance to prevent drops from becoming too large and dropping off the wall due to the poor wetting action. The cross drain strips are bonded to the bottom of each partition except the lowest, and the combined height of the drain strips and the vertical spacer ribs sets the spacing between the partitions. The stack of partitions with feed strips is held together with side and end extrusions that are just slid on, but otherwise not attached, so that different level partitions can expand different amounts without curving or buckling the structure. The drops condensing on the upper surfaces of each effect chamber drain into the cross feed strips and drain off the left end into a 1 inch wide feed strip (within two taller ribs) which drains at the left bottom. A 1-inch wide strip next to the distilled water drain strip is not fed source water, and a deep notch is cut into this strip to separate excess feed water from the distilled water. A catch basin with a partition now separately collects the source runoff and the distilled water.
This last design has the additional advantage of being able to be made into different numbers of effects if desired. It also allows the system to be disassembled to clean or replace bad portions.
There are many possible uses for a solar distillation system with the features of the present approach. The relative compactness and high productivity make it ideal for producing a small quantity of pure cooking and drinking water for a home. The lightness and simplicity of use make it ideal for a temporary or portable use system.
The present invention can easily be adapted to create a method for managing a partially, or even a fully self-contained home water supply system using an array of panels. Eighteen inches of water per year collected on a 1,200 square foot home area averages 40 gallons per day. This can be simply collected with a roof drain system and a storage tank. Rain generally produces this much or more except in desert areas. In order to average producing 140 gallons/day of solar distillations, you would need about 700 square feet of panels mounted on a south-facing roof (in the Northern Hemisphere). This is half of a 1200 square foot home area at a slant angle of 30 degrees. The supply water is a combination of rainwater collected from the roof and recaptured cooking and wash water. Flush water is lost (it can go to a septic tank in a fully independent system), but the rainwater replaces the loss. The distilled water comes out warm, so heating costs can also be reduced. It is thought the economics and convenience of such an approach would be particularly attractive in some regions of limited pure water supply.
It is thought that the thermal gradient driven regeneration multiple effect system described has design features that have not been used for any previous version of solar water purification system. The solar distillation system can also be used to distill other liquids compatible with the structure such as ethyl alcohol. In addition to distillation, the products come out hot. Another variation would use solar panels as the dark surface at the bottom of the system. The solar cells would convert part of the energy to electrical power, while the thermal heating would still be used for distillation.
Further objects, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.